Selenium compound useful as a brightener in a copper plating bath



United States Patent 3,497,530 SELENIUM COMPOUND USEFUL AS A BRIGHTENER IN A COPPER PLATING BATH Paul W. Moy, Cleveland, Ohio, assignor to Kewanee Oil Company, Bryn Mawr, Pa., a corporation of Delaware No Drawing. Original application Sept. 21, 1967, Ser. No. 669,401. Divided and this application May 5, 1969, Ser. No. 822,021

Int. Cl. C07d 99/.00

US. Cl. 260-3453 2 Claims ABSTRACT OF THE DISCLOSURE The compound of the formula and its use as a brightener in copper electroplating from cyanide baths.

This application is a divisional application of U.S. Ser. No. 669,401, filed Sept. 21, 1967. This invention relates to a new composition of matter and the use of such composition in electroplating from copper cyanide baths, more particularly to a bath composition which is particularly adapted to produce bright coatings of soft, ductile electroplates of copper.

Numerous attempts to obtain such bright coatings have been made. For instance, it has been proposed to use alkali metal selenite as an addition agent to an electrolytic bath in relatively large amounts. Such baths had a number of disadvantages in that the selenites tended to break down with a resultant adverse effect on the bright plating range. During the electrolysis because of breakdown of the selenites, the anodes became blackened, forming insoluble compounds which were loosened from the anodes and tended to co-deposit with the plated metal, resulting in rough deposits which were commercially unusable. Also in commercial operation, due to the resulting very narrow bright plating range, non-uniform appearance of the de posits resulted, and the deposits were in many cases insufiiciently bright so that butfing was necessary.

Another teaching of the prior art may be found in U.S. Patent 2,770,587, in which selenium compounds having a valence of 2 are claimed as brighteners in alkaline copper plating. The use of selenium compound having a Valence of 2 improved the brightness of the deposit over a wider current density range but in comparison with the instant invention the operable current density range is much less desirable.

The present invention is intended and adapted to overcome the difiiculties and disadvantages inherent in prior electro-plating baths of the type described. It is among the objects of the present invention to provide a bath composition in which a relatively small amount of addition agents is introduced resulting in clearly increased brightness with the plated surface, with a wider more uniform plating range.

It is also among the objects of the present invention to provide a bath composition which results in a plating which is brilliant, soft and ductile and which may be 3,497,530 Patented Feb. 24, 1970 "ice The selenium compound of the instant invention is the reaction product formed on reacting acetyl acetone and selenium dioxide. This reaction takes place on contacting the two reactants at a moderate temperature, for example, 70 C., approximate stoichiometric amount of the reactants are preferably employed, i.e., two moles of acetyl acetone per mole of selenium dioxide.

A typical preparation of this compound of the instant invention would be to charge a half liter three neck round bottom flask equipped with thermometer and reflux condensor in which is placed grams of acetyl acetone and 10 grams of selenium dioxide. The mixture is then heated while stirring to about 70 C. The reaction is completed in a very few minutes and the reaction product is cooled to room temperature and has the appearance of an orange to red liquid. This material can be used directly as a plating addition agent or it can be further purified by filtering to remove any trace quantities of metallic selenium and distilled in vacuo to remove excess acetyl acetone and traces of another component which is apparently formed by the reaction in trace quantities. The trace quantitles of this other component is probably 2,3,4-pentanetrione and can be detected by gas chromatography. After this purification the residue is a thick red oil which on standing solidifies to a dark orange mass. After multiple recrystallizations from benzene the compound of the instant invention is a bright yellow crystalline solid having a melting point of 6970 C. Results of the elemental analysis, molecular weight and infrared data confirmed the structure given above.

The compound of the instant invention is operable as a brightener in an alkaline copper plating bath at concentrations between about 0.005 and 0.5 grams per liter. The preferred concentration range for the compound of the instant invention in the copper plating bath is between 0.01 and 0.2 grams per liter. In this preferred range there is very little visible diiference in deposit appearance throughout the range which is advantageous in that the exact concentration is not highly critical.

The use of the additive of the instant invention produces a brightening effect with all of the known alkaline copper plating baths. Such alkaline copper plating baths are typified by the plating baths given in the examples that fol ow. These alkaline copper baths operate at temperatures ranging from about F. to about F. and the current density can vary widely as can be discussed later in detail. In the following examples, all proportions are in grams per liter and selenium compound refers to the compound of the instant invention as dis cussed supra.

3 EXAMPLE I These copper plating tests were conducted in a Hull cell of 50 cc. capacity. Brass Hull cell panels (2 /2 in. x 4 in.) were used as the plating substrate and normal agitation was used during plating. Plating was carried out at 3 amps for 5 minute period and the bath temperature was maintained at a temperature between 150- 160 F. The bath compositon was as follows:

Copper cyanide (CuCN) 75 Free potassium cyanide (KCN) 17 Potassium hydroxide (KOH) 25 Potassium carbonate (K CO 50 Selenium compound 101-.1

The resulting electroplate was semibright in the current density range 05 a.s.f., fully bright from 5120 a.s.f. and dull from 120l40 a.s.f.

EXAMPLE II Procedure of Examp e I was followed and the only variation in compositon was that 0.01 grams per liter dithioammelide was added to the bath. A fully bright deposit was obtained throughout the current density range of from 0-120 a.s.f.

EXAMPLE III Following the procedure of Example I, a bath of the following composition was tested.

Copper cyanide (CuCN) 75 Free sodium cyanide (NaCN) 12 Sodium carbonate (Na CO 35 Sodium hydroxide (NaOH) 20 Selenium compound 0.0l0.1

The resulting copper electroplate was semibright in the current density range of 0-3 a.s.f., fully bright from 3-70 a.s.f. and dull from 70-140 a.s.f.

EXAMPLE IV The procedure using the bath of Example III was repeated with the further addition of 0.01 grams per liter dithioammelide. This resulted in a fully bright deposit of copper from 070 a.s.f.

As will be noted from Examples II and IV, the addition of a small amount of dithioammelide improves the brightness in the extremely low current density areas. The use of dithioammelide and its equivalents are disclosed in detail in US. Patent No. 2,862,861 and describe the operable dithioammelide or its equiva ents as a thio substitu'ted six member heterocyclic ring compound wherein the members of the ring comprise 13 nitrogen atoms and the balance carbon atoms, each nitrogen atom forming a part of a separate azomethine group and being connected to two carbon atoms, at least one carbon atom of one azomethine group carrying a substituent of the class consisting of thiolmercaptide and alkylthiol. Typical examples falling under this description are 2,4,6-trimercapto triazine, dithioammelide, thioammeline, 4,6-diamino-2- mercapto pyrimidine, 4,6-diamino-2-merthyl mercapto pyrimidine, 2,4-dimercapto pyrimidine, 4-amino-6-hydroxy 2 mercapto pyrimidine, -6,amino 4 hydroxy 2- methyl mercapto pyrimidine, 4,hydroxy 2 mercapto 6- methyl pyrimidine, and 2-mercapto pyridine. All of these compounds are useful in conjunction with the compound of the present invention to improve the brightness of copper electroplate in the extremely low current density areas. However, care should be taken to hold the concentration of these heterocyclic compounds below 0.05 grams per liter if a dull electroplate is to be avoided in the medium current density areas. However, if all plating is done in the low current density range up to 10 grams per liter of these compounds can be used to effect a fully bright plate in the extremely low current density areas. Preferably, however, the concentration of dithioammelide is held between 0.001 and 0.05 grams liter.

A broad range of copper cyanide concentration may be employed with the brightening agents of the invention, and, in this respect, the copper cyanide concentration may range from about 45 to about 255 grams per liter. When no agitation is employed, best results are obtained when the concentration is from about 100 grams per liter to about 120 grams per liter. When the concentration falls be ow about 100 grams per liter it has been found that the brightness is adversely affected, and that below a concentration of about grams per liter only dull deposits result in the absence of agitation. In general, an increase in the copper cyanide concentration above about 120 grams per liter does not appreciably affect the brightness of the deposit obtained in non agitated solutions, and concentrations as high as about 225 grams per liter may be employed. On the other hand, when the plating solution is agitated, it has been found that the preferred copper cyanide concentration should be materially reduced for the obtainment of optimum resu ts. Thus, when vigorous agitation is employed, a copper cyanide concentration ranging generally from about 55 to about 85 grams per liter has been found best. With vigorous agitation as heretofore described it has been found, generally, that the brightness of the copper deposit is adversely affected when the concentration of copper cyanide falls outside a range which extends from about 45 to about grams per liter. When the solution is agitated, therefore, it will be found that the brightness contributed by the disclosed compound is seriously affected unless the copper cyanide concentration is adjusted to compensate for the degree of agitation, or vice versa.

As is well known to those skilled in the art of copper cyanide plating, a complex potassium cuprous cyanide compound is formed between the potassium cyanide and copper cyanide. The actual formula of the complex varies according to the temperature conditions of the solution among other things. In this regard, the potassium cyanide concentration excluding the free potassium cyanide generally amounts to about 1.46 times the amount of copper cyanide employed. Thus, for a bath containing copper cyanide in amounts between about 45 and 225 grams of copper cyanide per liter should contain potassium cyanide excluding the free cyanide, of from about 65 to about 325 grams per liter to form the complex. Similarly for a solution having amounts of copper cyanide ranging between about and grams of copper cyanide per liter the potassium cyanide concentration, excluding the free cyanide, should be between about and grams of potassium cyanide per liter. The potassium cyanide concentration is thus based upon a potassium copper cyanide complex having the approximate formula K Cu(CN) It is necessary in copper cyanide solutions to have sufficient cyanide present to form a complex, otherwise, unstable conditions result. In this regard, there must 'be an excess of free cyanide such as sodium or potassium cyanide to insure that the complex is formed.

It has been found that generally greater quantities of free cyanide must be present when the solution is agitated than when the solution is not agitated. For example, when the solution is not agitated, it has been found that when the concentration of the free cyanide falls below about 2.5 grams of free potassium cyanide per liter, a dull deposit results. A similar dullness is detected above a free cyanide concentration of about 25 grams per liter. The best results in non-agitated solutions are obtained when the free cyanide concentration ranges from about 4 grams per liter to about 10 grams per liter. On the other hand, when vigorous agitation is employed as described heretofore, it has been found that best results are obtained when the free cyanide concentration ranges generally from about 9 to about 12 grams per liter. With vigorous agitation excessive anode polarization occurs, and a thick black coating is formed on the anode surface when the free cyanide concentration falls below about 6 grams per liter. Whenthe concentration of free cyanide exceeds about 13 grams per liter, the cathode efliciency is decreased appreciably. On the other hand, Where no agitation is employed, it is found that the free cyanide concentration may fall as low as about 2.5 grams per liter, below which, however, excessive anode polarization occurs, and a thick black coating is formed on the anode surface. Usually the free cyanide concentration in excess of about 17 or 18 grams per liter is not recommended since the cathode efliciency decreases above this concentration due to excessive gassing, and only dull deposits are obtained above about 25 grams per liter of free cyanide. It will be apparent because of the necessity for free cyanide that the maximum limiting quantity of potassium cyanide is about 350 grams per liter when the copper cyanide concentration is about 225 grams per liter. The 350 grams per liter of potassium cyanide being the sum of the potassium cyanide associated with the copper cyanide and the free cyanide.

In preparing the basic solution, copper cyanide is preferably added to an aqueous solution of potassium or sodium cyanide in the desired amounts, and potassium or sodium hydroxide thereafter added to obtain the desired operating pH range. In general, a wide range of pH may be tolerated in the solution with optimum results being obtained in the pH range between about 11.5 and 12.5 in the case where there is no agitation, and in a pH range between about 12 and 13.5 in the case where there is vigorous agitation. Good results may be obtained in either case, however, at pHs ranging from about 9 to 14.

To buffer the solution against changes in pH, potassium citrate in amounts generally ranging from about 35 to about 75 grams per liter has been found suitable. Best results have been obtained when the potassium citrate is employed in amounts ranging from about 45 grams per liter to about 65 grams per liter. It will be apparant that it is not essential that potassium citrate be employed as a buffer or, in fact, that any buffer be utilized in the plating solution. Other buffers such as Rochelle salts may be employed also, and, in this regard, the Rochelle salts may be employed generally in amounts ranging from about to about 55 grams per liter. Usually the Rochelle salts have been found to be more effective in the agitated solutions than in the non-agitated solution.

With respect to the temperature of the plating solution during the plating process optimum results in nonagitated solutions may be obtained over a somewhat wider temperature range than in the case where vigorous agitation is employed. For example, temperatures ranging from about 140 F. to about 160 F. may be employed without agitation of the solution, whereas, a temperature from about 150 F. to about 160 F. is preferably employed when there is vigorous agitation. Again, it will be apparent that the ranges set forth herein with respect to an agitated and non-agitated solution are principally illustrative of the invention, and that with varying degrees of the agitation the concentrations and operating conditions will vary so far as their optimum is concerned. Between the extreme case of agitation illustrated, and the other extreme where there is relatively no agitation, solution temperatures of from about 130 F. to about 185 F. may be employed. Although temperatures as low as 130 F. have been successfully employed, they are not strongly recommended since the brightness of the plate obtained tends to diminish if the temperature of the plating solution falls below about F. in the case where agitation Is not employed. On the other hand, when temperatures in excess of about F. are employed without agitation slightly higher current densities should be utilized. The bright plating current density range tends to increase somewhat when higher temperatures are employed. In low current density areas the brightness usually diminishes when the temperature is increased much above 160 F. without a compensating increase in the average current density employed. It is pointed out, however, that temperatures as high as about F. have been employed with success utilizing the brightening agent described herein.

Broadly, current densities up to about 120 amps per square foot of cathode surface area may be utilized according to the invention. Interrupted current appears to permit a wider and somewhat higher range of current densities and is usually preferred since it aids in eliminating polarization, and further minimizes film deposits on the anode. Suitable cycles for the use of interrupted current may have an on time of up to about 90 seconds and an off time of from about 5 to about 50% of the on time. With a continuous current, optimum results in the form of maximum brightness have been obtained in non-agitated solutions when the current density ranges from about 10 to about 35 amps per square foot of cathode surface area, whereas, with an interrupted current, one may use from about 10 to about 40 amps per square foot without agitation. On the other hand, when vigorous agitation and a continuous current are employed, it has been found that a preferred range of current densities from about 10 to 20 amps per square foot is best, whereas, with an interrupted current, the range is broadened out and may be raised to from about 10 to 60 amps per square foot.

I claim:

1. A compound of the formula 2. The method of making a brightening additive for use in the plating a bright copper deposit from alkaline copper electroplating baths comprising mixing acetyl acetone and selenium dioxide at a slightly elevated temperature and recovering the material produced at the conclusion of the reaction.

References Cited UNITED STATES PATENTS 2,770,587 11/1956 Moy 204-52 US. Cl. X.R, 204-52 

